Electroluminescence devices which utilize electroluminescence show high self-distinguishability because of the self-emission and are excellent in impact resistance because they are completely solid devices. Therefore, electroluminescence devices have been attracting attention for application as light emitting devices in various types of display apparatus.
The electroluminescence devices include inorganic electro-luminescence devices in which an inorganic compound is used as the light emitting material and organic electroluminescence devices in which an organic compound is used as the light emitting material. Organic electroluminescence devices have been extensively studied for practical application as a light emitting device of the next generation because the applied voltage can be decreased to a large extent, the size of the device can be reduced easily, consumption of electric power is small, planar light emission is possible and three primary colors are easily emitted.
As for the construction of the organic electroluminescence device, the basic construction comprises an anode/an organic light emitting layer/a cathode. Constructions having a hole injecting and transporting layer or an electron injecting layer suitably added to the basic construction are known. Examples of such construction include the construction of an anode/a hole injecting and transporting layer/an organic light emitting layer/a cathode and the construction of an anode/a hole injecting and transporting layer/an organic light emitting layer/an electron injecting layer/a cathode.
Recently, organic EL devices for display apparatuses have been developed actively. In particular, a device which can emit white light is the major target of the development. An organic EL device emitting white light can be used as the light source for single color displays and back light. Moreover, an organic EL device emitting white light can be used for full color display when color filters are attached to a display apparatus.
For examples, an organic EL device emitting white light is disclosed in the U.S. Pat. No. 5,503,910, in which a laminate of a light emitting layer emitting blue light and a light emitting layer emitting green light is used as the layer of a light emitting medium and a fluorescent compound emitting red light is added to the layer of a light emitting medium. An organic EL device emitting white light is disclosed in the U.S. Pat. No. 5,683,828, which has a layer of a light emitting medium obtained by adding a complex compound containing boron which is a fluorescent compound emitting red light to a light emitting layer emitting bluish green light. An organic EL device emitting white light is disclosed in Japanese Patent Application Laid-Open No. Heisei 10(1998)-308278, which has a light emitting medium obtained by adding a benzothioxanthene derivative which is a fluorescent compound emitting red light to a light emitting layer emitting bluish green light.
However, the device disclosed in the U.S. Pat. No. 5,503,910 has an efficiency of light emission as low as about 1 lumen/W and a life as short as about 1,000 hours although white light is emitted. The device disclosed in the U.S. Pat. No. 5,683,828 has an efficiency of light emission as low as about 2.6 cd/A although white light is emitted. The device disclosed in Japanese Patent Application Laid-Open No. Heisei 10(1998)-308278 has an efficiency of light emission as low as about 1 lumen/W although white light is emitted. No devices sufficiently satisfy the requirements for practical application with respect to the efficiency of light emission and the life.